A known technique for separating constituents A and B in solution in a liquid mixture consists in injecting it into a “chromatographic column” subjected to a centrifugal force, so designed that one of the liquid phases can be percolated into the other liquid phase and vice versa (chromatography referred to as CCC or CPC).
In practice, as shown notably in patents FR-2,791,578, U.S. Pat. No. 4,551,251, U.S. Pat. No. 4,877,523 or U.S. Pat. No. 4,857,187, this type of system comprises one or more stacks of discs driven in rotation. Each one comprises (FIG. 4), in the thickness and over the entire periphery thereof, a succession of cells CE arranged in a radial or oblique direction and connected in series by a set of circuits of fine winding channels B at the ends of each cell. The circuits of all the discs communicate with one another. Cells CE and their communication circuits B are filled with a stationary liquid phase that is kept in place by the centrifugal force and with another mobile liquid phase that percolates through the stationary phase.
Rotation of the stack creates a high centrifugal acceleration field that allows the stationary liquid phase to remain fixed whereas the mobile phase circulates in a mode referred to as ascending (FIG. 1A) if it is lighter than the stationary phase, and in descending mode (FIG. 1B) if it is heavier.
The chromatographic process, i.e. partition of the molecules to be purified between the two liquid phases, takes place in each cell, and mass transfer is favoured by a good dispersion of the mobile phase flowing from the channel into each cell.
To obtain better separation, it is possible for example, as described in patent application FR-03/08,076, to inject the feedstock at an intermediate point of the chain of cells making up the column, and to carry out alternate cycles of two stages, with a first stage during a first time interval where a lighter solvent is injected through a first end of the device and a first component is collected at a second end of the device, and a second stage during a second time interval where a heavier solvent is injected through the second end of the device and a second constituent is collected at the first end. The respective lengths of the first and of the second stage and/or the rates of injection of the lighter solvent and of the heavier solvent are adjusted according to the constituents of the mixture so as to obtain optimum separation.
Whatever the shape thereof, each CPC cell can be characterized (FIG. 2A) by its length L, measured in a radial direction (or close to a radial direction), by its width l, measured in a direction normal (or close) to the radial direction, these first two quantities being measured in a plane normal to axis of rotation Ω, and by its thickness e, measured along a direction parallel (or close) to the axis of rotation.
It can be easily checked that selection of these three dimensions for a given cell volume has a great impact on the separation efficiency obtained. The problem of selecting the right dimensions for the cells arises when an efficient separation system is to be designed, and also when the size thereof is to be changed to switch from an analytic type installation to an industrial type installation, or vice versa, while keeping the same efficiency.